Resonant MEMS sensors operating in harsh fluidic environments demand high sensitivity and structural robustness, which conventional silicon devices often fail to provide. This paper proposes the design and comprehensive multiphysics calibration of a highly sensitive, photothermally actuated MEMS sensor utilizing a porous Cu-Ni functionally graded material (FGM) microcantilever. Rather than traditional power sensing, this study pioneers its application in trace mass sensing, providing a rigorous mass-loading calibration curve ([[EQUATION]]). The device exhibits an exceptional trace mass sensitivity of 25.5 Hz/ng in air and maintains a highly linear responsivity of 8.5 Hz/ng even in heavily damped aqueous environments. Through rigorous topology optimization, a central (O-type) pore distribution is identified as the optimal configuration to capture target molecules near the physical neutral plane, successfully overcoming the traditional mass-stiffness penalty. Furthermore, fluid-structure interaction analyses confirm the sensor's operational integrity in highly viscous fluids (e.g., gasoline and water), retaining significantly higher Q-factors than conventional counterparts. By validating these device-level metrics using the Modified Couple Stress Theory (MCST) and Temperature-Dependent Material Properties (TDMP), the proposed Cu-Ni FGM sensor offers a robust, high-stability paradigm for next-generation in-situ mass detection in harsh environments.